Fig. 275.—Thecocystis saeculus. × 6. a. Anus; m, mouth; p, madreporite(?) (After Jaekel.)

CLASS III. CARPOIDEA

Pelmatozoa with a well-developed stem; body bilaterally compressed; only two rays apparently developed. These are indicated only by grooves radiating from the mouth; but in some cases slight horn-like outgrowths of some of the plates of the calyx may support prolongations of the grooves.

This group, which, like the foregoing, commences in the Cambrian, is perhaps more primitive than the Thecoidea in showing less influence of the water-vascular system on the skeleton; but in the presence of a differentiated stem and the development of only two rays, it is more differentiated. The anus is on one of the flat sides, covered with a flat plate acting as a valve. The members of this group were formerly confounded with Cystoidea, from which they differ in the absence of the characteristic pores. Trochocystis, the genus figured, is devoid of any horn-like outgrowths of the calyx.

Fig. 276.—Trochocystis bohemicus, viewed from two sides. o, Mouth. (After Jaekel.)

CLASS IV. CYSTOIDEA

Pelmatozoa with respiratory organs in the form of "diplopores" or "pore-rhombs." In a great many cases there is a stalk, but in other cases this is atrophied, and the animal is attached by the base of the calyx. The radial canals run for a shorter or longer distance over the calyx, but the plates of the calyx themselves are not modified for them. Either they run in simple grooves, or they are protected by a special series of plates lying above the plates of the calyx. The terminal portions of the radial canals are in all cases free, supported by unbranched arms consisting usually of a double row of ossicles. These arms are termed "fingers."

It will be gathered from the description just given that the fingers and the respiratory organs distinguish Cystoidea from the two foregoing classes. Formerly this class was a lumber-room in which were placed all primitive irregular Pelmatozoa. The labours of Jaekel[[516]] have, however, dispelled the mist which enveloped this group, and in his monograph all that can be extracted both from superficial examination and dissection of these fossils is contained. It seems possible to the present author that the class may eventually require to be divided into two, corresponding to the two main divisions which Jaekel recognises, viz. Dichoporita, with pectinated rhombs, and Diploporita, with diplopores.